Three major joining technologies exist for aerospace composite structure: mechanical fastening; adhesive bonding; and welding (also called, fusion). Both mechanical fastening and adhesive bonding are costly, time consuming assembly steps that introduce excess cost even if the parts that are assembled are fabricated from components produced by an emerging, cost efficient process. Mechanical fastening requires expensive hole locating, drilling, shimming, and fastener installation, while adhesive bonding requires complicated surface pretreatments.
In contrast, thermoplastic welding features the ability to join thermoplastic composite components at high speeds with minimum touch labor and little, if any, pretreatments. In our experience, the welding interlayer, called a susceptor, also can simultaneously take the place of shims required in mechanical fastening. As such, composite welding holds promise to be an affordable joining process. For "welding" thermoplastic and thermoset composite parts together, the susceptor functions as a hot melt adhesive. If fully realized, the thermoplastic-thermoset bonding will further reduce the cost of composite assembly.
There is a large stake in developing a successful induction welding process. Its advantages versus traditional composite joining methods are:
reduced parts count versus fasteners PA1 minimal surface preparation, in most cases a simple solvent wipe to remove surface contaminants PA1 indefinite shelf life at room temperature PA1 short process cycle time, typically measured in minutes PA1 enhanced joint performance, especially hot/wet and fatigue PA1 permits rapid field repair of composites or other structures
Because magnetic field strength decreases exponentially as the fields propagate, the plies of a graphite or carbon fiber reinforced resin matrix composite structure closest to an induction coil will always be substantially hotter than the remote plies at the center of the structure, when relying upon heating through induced currents in the fibers. The fibers however, are relatively poor conductors so fields of high strength are required to achieve any significant heating. If such fields are used, then the fibers closest the induction coil (i.e., magnetic field source) heat most, and actually must overheat to obtain adequate heating at the bond line. To avoid overheating of the outer plies and to insure adequate heating of the inner plies, we use a susceptor having significantly higher conductivity than the fibers. The susceptor has a mesh pattern to allow the adhesive to bond between the prefabricated elements of the composite assembly through the susceptor. The susceptor selectively peaks the heating at the bond line.
Eddy currents that the magnetic field induces in the susceptor, however, produce a higher current density at the edges of the susceptor than in the center, which itself can produce damaging overheating at the edges of the assembly or, at the opposite extreme, underheating the center when the power is controlled to avoid edge overheating.
In our thermoplastic welding process, we apply an electromagnetic induction coil to one side of the assembly to heat the susceptor through the induced eddy currents and to melt the thermoplastic material so than when it resolidifies the elements bond together. To achieve uniform heating, the susceptor design must be tailored to control the current density the induction coil induces. The heating we achieve is directly proportional to the power which the susceptor dissipates. That power is given by the common electromagnetic equation for power loss in a resistor: EQU P=(J.sup.2)(R)
wherein P is the power density, J is the eddy current density, and R is the resistance or impedance through which current flows. Therefore, to counter higher current density at the edges (i.e., a higher "J"), we tailor the susceptor near the edges to lower the resistance there relative to the center so that the product (i.e. the power and the heat) remains relatively constant across the width of the susceptor.
Prior art thermoplastic welding processes are illustrated in U.S. Pat. Nos. 3,996,402 and 4,120,712. In these processes, the susceptors have a regular pattern of openings so the resistance near the edges is identical to the resistance in the center. Therefore, these conventional susceptors produce nonuniform heating from center to edge as the current density increases toward the edge.